This research investigates how and under what circumstances children and their social partners collaborate in joint problem solving, and how children later make use of the collaborative processes. The studies investigate the concept of guided participation (Rogoff, 1990), which builds on Vygotsky's sociohistorical theory emphasizing children's interactions with others. The proposal involves 4 continuing studies and 7 new studies examining the processes and effects of children's collaboration with adults and with peers in situations varying in the partners' expertise, involvement, and 'ownership' in the activity. Some of the proposed studies address issues raised by the research to date that show that working with adults on errand planning tasks yields advances in 9-year-olds' independent planning in such tasks, compared with working with peers -- apparently due to the children's greater guidance and participation in more sophisticated planning during collaborative planning with the adults. Studies with 4- and 5-year-old children did not show advantages of collaborative planning for later individual planning, except when the partners managed to share in decision making. Other proposed studies investigate the generality and variations in 1- to 9-year-old children's guided participation. The studies add memory and logical tasks to see whether the patterns observed in errand planning problems appear with problems in which joint reference may be easier or in which shifts in perspective are necessary. They examine collaborative problem solving under circumstances in which adults may be less in charge or less focused on teaching (partners are involved in competing activities; adults are depressed; families come from a community in which children are responsible for learning rather than adults responsible for teaching), and children may be more on their own ground and in charge of learning (in computer games and play; or coming from a community in which children are responsible for learning). A focus of the work is children's active efforts to observe and participate and manage their involvement.